I wired in a double din radio in my 300TE with using the existing speaker wiring. The stock amp didn't work any ways (was disconnected) and was mounted in the rear. So I piggybacked two more speakers off of that and mounted them in the rear.
It's pushing 8 speakers off the stereo HU's amp and sounds great.
The GPS Garmin C550 can be used as an MP3 player, blue tooth phone will pause the music and put your call through the stereo
The stereo also illuminates in different colors.